Arena help

The 3 most important things to know about AI. If a unit can attack you within its starting range (I mean the range than it has when it starts its turn) it will always attack, and it will always go first for whichever they kill or doesn't kill them. If they're far away, it can be difficult to predict, but if they one-round a unit, they'll usually approach that unit. And finally, if the unit has an assist skills and can use the assist skill, it will ALWAYS use the assist.

For a defense team, I'd just make a team of all of the same color and then a dancer or singer. This has the best chance to accidentally trip your opponent up when you're on defense, especially down in tier 16 and 17. Also, don't lead with a horse unit or a dancer because it will make less experienced players avoid it. Lead with something that doesn't shout "OVERPOWERED" while at the same time doesn't shout "BAIT"

Other things to know about arena is that mages are really good units to use, but they usually don't score as well as melee units do unless they're heavily merged. If you're hurting for arena scores, higher sp skills give higher points in arena, and by the time you have 4 fleshed out 5* units with a bonus unit, you'll hit 680+ per match assuming you go deathless. Also you NEED to be able to check Reinhardt and Brave Lyn. They are everywhere in the arena right now and it's hard for me to find a team where I don't face one or both. A good answer to those is a Triangle Adept Gronnraven tome user.

For arena score
--Use at least 1 season unit if you can use 2 is better. 1 of the starter units always are season unit.
--Max level your units skills if possible.

For offence team
--Try to focus more on your team synergy(but without ignore best units),
--Be sure to add move skills like reposition, draw back or swap they can help you a lot to stay out of enemy range and you can be able to "Hit and Run"

A few tips off the top of my head from someone who hangs out in 18 and occasionally 19...

Team building: do not underestimate the power of dancers and movement supports (reposition, draw back, swap). Also make sure you have someone who can nuke people from range (Nino, Reinhardt, any brave bow archer). If you have anyone with distant or close counter (Takumi, Hector, Ryoma, A!Tiki, Nowi, Xander, Camus, Ike) they can be a big help too.

Dealing with AI: If no unit is in range for the AI to attack, the AI will move toward the unit that gives them an advantageous type matchup. My arena performance improved dramatically when I figured this out. Also, the AI uses movement skills poorly, because they tend to use them on units who've already moved.

General tips: take a good look at the enemy skills before you start, and be extra wary if you see anyone with Wings of Mercy or Escape Route. Also make sure you account for extra movement range if the enemy has a dancer.

Might consider a mixed cavalry + infantry/armor team. That gives you the power and versatility to win with the BST to give you a decent score. And you can also be picky with what battles you accept going into. Surrendering before choosing a difficulty doesn't reset your score. I've use these tips to get to the border of maybe being able to stay in tier 20 this season as a F2P.